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alistair23-linux/fs
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Davide Libenzi bcd0b235bf eventfd: improve support for semaphore-like behavior 
People started using eventfd in a semaphore-like way where before they
were using pipes.

That is, counter-based resource access.  Where a "wait()" returns
immediately by decrementing the counter by one, if counter is greater than
zero.  Otherwise will wait.  And where a "post(count)" will add count to
the counter releasing the appropriate amount of waiters.  If eventfd the
"post" (write) part is fine, while the "wait" (read) does not dequeue 1,
but the whole counter value.

The problem with eventfd is that a read() on the fd returns and wipes the
whole counter, making the use of it as semaphore a little bit more
cumbersome.  You can do a read() followed by a write() of COUNTER-1, but
IMO it's pretty easy and cheap to make this work w/out extra steps.  This
patch introduces a new eventfd flag that tells eventfd to only dequeue 1
from the counter, allowing simple read/write to make it behave like a
semaphore.  Simple test here:

http://www.xmailserver.org/eventfd-sem.c

To be back-compatible with earlier kernels, userspace applications should
probe for the availability of this feature via

#ifdef EFD_SEMAPHORE
	fd = eventfd2 (CNT, EFD_SEMAPHORE);
	if (fd == -1 && errno == EINVAL)
		<fallback>
#else
		<fallback>
#endif

Signed-off-by: Davide Libenzi <davidel@xmailserver.org>
Cc: <linux-api@vger.kernel.org>
Tested-by: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2009-04-01 08:59:20 -07:00
..
9p vfs: simple_set_mnt() should return void 2009-03-27 14:44:03 -04:00
adfs constify dentry_operations: misc filesystems 2009-03-27 14:44:00 -04:00
affs constify dentry_operations: misc filesystems 2009-03-27 14:44:00 -04:00
afs proc 2/2: remove struct proc_dir_entry::owner 2009-03-31 01:14:44 +04:00
autofs constify dentry_operations: autofs, autofs4 2009-03-27 14:44:00 -04:00
autofs4 constify dentry_operations: autofs, autofs4 2009-03-27 14:44:00 -04:00
befs
bfs
btrfs fs: fix page_mkwrite error cases in core code and btrfs 2009-04-01 08:59:14 -07:00
cifs proc 2/2: remove struct proc_dir_entry::owner 2009-03-31 01:14:44 +04:00
coda constify dentry_operations: misc filesystems 2009-03-27 14:44:00 -04:00
configfs constify dentry_operations: configfs 2009-03-27 14:44:03 -04:00
cramfs
debugfs
devpts Merge code for single and multiple-instance mounts 2009-03-27 14:44:04 -04:00
dlm dlm: fix length calculation in compat code 2009-03-11 12:23:59 -05:00
ecryptfs constify dentry_operations: ecryptfs 2009-03-27 14:44:01 -04:00
efs
exportfs
ext2 ext2: Zero our b_size in ext2_quota_read() 2009-03-26 02:18:38 +01:00
ext3 Merge branch 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-quota-2.6 2009-03-27 14:48:34 -07:00
ext4 mm: page_mkwrite change prototype to match fault 2009-04-01 08:59:14 -07:00
fat constify dentry_operations: FAT 2009-03-27 14:44:01 -04:00
freevxfs fs/Kconfig: move vxfs out 2009-01-22 13:15:58 +03:00
fuse mm: page_mkwrite change prototype to match fault 2009-04-01 08:59:14 -07:00
gfs2 mm: page_mkwrite change prototype to match fault 2009-04-01 08:59:14 -07:00
hfs constify dentry_operations: misc filesystems 2009-03-27 14:44:00 -04:00
hfsplus constify dentry_operations: misc filesystems 2009-03-27 14:44:00 -04:00
hostfs constify dentry_operations: misc filesystems 2009-03-27 14:44:00 -04:00
hpfs constify dentry_operations: misc filesystems 2009-03-27 14:44:00 -04:00
hppfs
hugetlbfs mm: reintroduce and deprecate rlimit based access for SHM_HUGETLB 2009-04-01 08:59:12 -07:00
isofs constify dentry_operations: misc filesystems 2009-03-27 14:44:00 -04:00
jbd jbd: fix return value of journal_start_commit() 2009-02-11 14:25:35 -08:00
jbd2 jbd2: Avoid possible NULL dereference in jbd2_journal_begin_ordered_truncate() 2009-02-10 11:15:34 -05:00
jffs2 [JFFS2] fix mount crash caused by removed nodes 2009-02-21 11:09:29 +01:00
jfs proc 2/2: remove struct proc_dir_entry::owner 2009-03-31 01:14:44 +04:00
lockd NLM: Fix GRANT callback address comparison when IPv6 is enabled 2009-03-10 20:33:20 -04:00
minix Update my email address 2009-03-22 11:28:37 -07:00
ncpfs constify dentry_operations: misc filesystems 2009-03-27 14:44:00 -04:00
nfs mm: page_mkwrite change prototype to match fault 2009-04-01 08:59:14 -07:00
nfs_common
nfsd Merge branch 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-quota-2.6 2009-03-27 14:48:34 -07:00
nls
notify fs: avoid I_NEW inodes 2009-03-27 14:44:05 -04:00
ntfs ntfs: remove private wrapper of endian helpers 2009-04-01 08:59:18 -07:00
ocfs2 mm: page_mkwrite change prototype to match fault 2009-04-01 08:59:14 -07:00
omfs fs/Kconfig: move omfs out 2009-01-22 13:15:58 +03:00
openpromfs
partitions Merge branch 'for-linus' of git://git390.marist.edu/pub/scm/linux-2.6 2009-03-26 16:04:22 -07:00
proc proc tty: remove struct tty_operations::read_proc 2009-04-01 08:59:10 -07:00
qnx4 fs/Kconfig: move qnx4 out 2009-01-22 13:15:59 +03:00
quota Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs-2.6 2009-03-27 16:23:12 -07:00
ramfs ramfs-nommu: use generic lru cache 2009-04-01 08:59:15 -07:00
reiserfs Merge branch 'proc-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/adobriyan/proc 2009-03-30 16:06:04 -07:00
romfs fs/Kconfig: move romfs out 2009-01-22 13:15:59 +03:00
smbfs constify dentry_operations: misc filesystems 2009-03-27 14:44:00 -04:00
squashfs Squashfs: Valid filesystems are flagged as bad by the corrupted fs patch 2009-03-12 03:23:48 +00:00
sysfs mm: page_mkwrite change prototype to match fault: fix sysfs 2009-04-01 08:59:14 -07:00
sysv constify dentry_operations: misc filesystems 2009-03-27 14:44:00 -04:00
ubifs mm: page_mkwrite change prototype to match fault 2009-04-01 08:59:14 -07:00
udf udf: Use lowercase names of quota functions 2009-03-26 02:18:36 +01:00
ufs Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs-2.6 2009-03-27 16:23:12 -07:00
xfs mm: page_mkwrite change prototype to match fault 2009-04-01 08:59:14 -07:00
aio.c aio: lookup_ioctx can return the wrong value when looking up a bogus context 2009-03-19 15:57:18 -07:00
anon_inodes.c constify dentry_operations: rest 2009-03-27 14:44:03 -04:00
attr.c vfs: Use lowercase names of quota functions 2009-03-26 02:18:35 +01:00
bad_inode.c
binfmt_aout.c
binfmt_elf.c elf core dump: fix get_user use 2009-02-06 17:34:07 -08:00
binfmt_elf_fdpic.c
binfmt_em86.c
binfmt_flat.c
binfmt_misc.c
binfmt_script.c
binfmt_som.c
bio-integrity.c block: add private bio_set for bio integrity allocations 2009-03-24 12:35:17 +01:00
bio.c block: add private bio_set for bio integrity allocations 2009-03-24 12:35:17 +01:00
block_dev.c fs: move bdev code out of buffer.c 2009-03-27 14:44:03 -04:00
buffer.c filesystem freeze: allow SysRq emergency thaw to thaw frozen filesystems 2009-04-01 08:59:17 -07:00
char_dev.c
compat.c fix setuid sometimes doesn't 2009-03-28 17:30:00 -07:00
compat_binfmt_elf.c
compat_ioctl.c Merge branch 'master' of master.kernel.org:/pub/scm/linux/kernel/git/davem/net-2.6 2009-03-01 21:35:16 -08:00
dcache.c cleanup d_add_ci 2009-03-27 14:43:57 -04:00
dcookies.c
direct-io.c
drop_caches.c fs: avoid I_NEW inodes 2009-03-27 14:44:05 -04:00
eventfd.c eventfd: improve support for semaphore-like behavior 2009-04-01 08:59:20 -07:00
eventpoll.c epoll: use real type instead of void * 2009-04-01 08:59:20 -07:00
exec.c fix setuid sometimes doesn't 2009-03-28 17:30:00 -07:00
fcntl.c Fix a lockdep warning in fasync_helper() 2009-03-30 08:00:24 -06:00
fifo.c
file.c
file_table.c Merge branch 'bkl-removal' of git://git.lwn.net/linux-2.6 2009-03-26 16:14:02 -07:00
filesystems.c
fs-writeback.c fs: new inode i_state corruption fix 2009-03-12 16:20:24 -07:00
generic_acl.c
inode.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs-2.6 2009-03-27 16:23:12 -07:00
internal.h fix setuid sometimes doesn't 2009-03-28 17:30:00 -07:00
ioctl.c Rationalize fasync return values 2009-03-16 08:34:35 -06:00
ioprio.c
Kconfig quota: Move quota files into separate directory 2009-03-26 02:18:35 +01:00
Kconfig.binfmt
libfs.c vfs: simple_set_mnt() should return void 2009-03-27 14:44:03 -04:00
locks.c
Makefile quota: Move quota files into separate directory 2009-03-26 02:18:35 +01:00
mbcache.c
mpage.c
namei.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs-2.6 2009-03-27 16:23:12 -07:00
namespace.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs-2.6 2009-03-27 16:23:12 -07:00
nfsctl.c
no-block.c
open.c vfs: Use lowercase names of quota functions 2009-03-26 02:18:35 +01:00
pipe.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs-2.6 2009-03-27 16:23:12 -07:00
pnode.c
pnode.h
posix_acl.c
read_write.c
read_write.h
readdir.c
select.c
seq_file.c cpumask: fix seq_bitmap_*() functions. 2009-03-30 22:05:11 +10:30
signalfd.c
splice.c
stack.c
stat.c
super.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs-2.6 2009-03-27 16:23:12 -07:00
sync.c Merge branch 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-quota-2.6 2009-03-27 14:48:34 -07:00
timerfd.c timerfd: add flags check 2009-02-18 15:37:53 -08:00
utimes.c
xattr.c
xattr_acl.c